The present invention relates to an improved sheet inverting method and apparatus for inverting sheets traveling through a machine, for example, a printing or photocopying machine.
As xerographic and other copiers and printers increase in speed and become more automatic, it is increasingly important to provide higher speed yet more economical, reliable and more automatic handling of both the print sheets being printed by the machine and the original document sheets being copied. It is also desired to accommodate sheets which may vary widely in size, weight, thickness, material, condition, humidity, age, etc. These variations change the beam strength or flexural resistance and other characteristics of the sheets. Yet, the desire for automatic and high speed handling of such sheets without jams, misfeeds, uneven feeding times, damaged sheets, smeared print or other interruptions increases the need for reliability of all sheet handling components. A sheet inverter is one such sheet-handling component with particular reliability and speed limitation problems.
Although a sheet inverter is referred to in the copier and printer art as an xe2x80x9cinverterxe2x80x9d, its function is not necessarily to turn the sheet over (i.e., exchange one face for the other). Its function may be to effectively reverse the sheet orientation in its direction of motion. That is, to reverse the leading edge and trailing edge orientation of the sheet. Typically in inverter devices, the sheet is driven or fed by feed rollers, conveyors or other suitable sheet driving mechanisms into a sheet-reversing chute. By then reversing the motion of the sheet within the chute and feeding it back out from the chute, the desired reversal of the leading and trailing edges of the sheet in the sheet path is achieved.
Depending on the location and orientation of the inverter in a particular sheet path, the reversal of the leading and trailing edges of the sheet may or may not also accomplish an inversion (turning over) of the sheet. In some applications, for example, where the xe2x80x9cinverterxe2x80x9d is located at the corner of a 90xc2x0 to 180xc2x0 bend in the copy sheet path, the inverter may be used to prevent inversion of a sheet and thereby maintain the same face of the sheet face-up before and after this bend in the sheet path. On the other hand, if the entering and departing path of the sheet, to and from the inverter, is in substantially the same plane, the inverter will invert the sheet. Thus, inverters have numerous known applications in the handling of either original document sheets or print sheets (collectively referred to herein as xe2x80x9cprint sheetsxe2x80x9d or simply xe2x80x9csheetsxe2x80x9d) to selectively maintain and/or change the sheet orientation.
Inverters are particularly useful in various systems for pre or post collation copying or printing, for inverting the original documents, or for maintaining proper collation of the print sheets. The facial orientation of the sheet determines whether it will be stacked in forward or reversed serial order. Generally, the inverter is associated with a by-pass sheet path and gate so that a sheet may selectively by-pass the inverter, in order to provide a choice of inversion or non-inversion. Inverters are also useful in inverting the sheet to print on and/or copy from both sides of the sheet for duplex copying and printing.
In one type of known reversing chute inverter, the sheet is fed into and then wholly or partially released from a positive feeding grip roller pair or input nip into the reversing chute. The sheet is then reacquired by a different feeding grip roller pair or exit nip and is driven in the reverse direction to exit the reversing chute. U.S. Pat. Nos. 3,944,212 and 4,078,789 are examples of tri-roll type reversing chute inverters. A tri-roll reversing chute inverter includes a set of three rollers (the tri-roll) that defines the input and exit nips of the inverter. A reversing pinch roll pair or reversing nip is located downstream of the tri-roll in the reversing chute. The reversing nip reverses the sheet""s direction of travel and feeds the sheet into the output nip.
In U.S. Pat. Nos. 3,944,212 and 4,078,789 cited above, the reversing nip is rotated in the reverse direction only and is maintained open as the copy sheet is fed into the reversing nip by the input nip. A sensor just downstream of the input nip senses when the trailing edge of the sheet has exited the input nip. When it is sensed that the sheet has exited the input nip, the reversing nip is activated to engage the sheet and drive the sheet in the reverse direction into the exit nip. For a brief period, between the time the sheet exits the input nip and is re-acquired by the reversing nip, the sheet is not positively gripped by any feed nips and is traveling freely under it""s own momentum.
The inverters disclosed in U.S. Pat. Nos. 3,944,212 and 4,078,789 open a gap in the reversing nip by forming a flat on the drive roller of the nip and stopping the drive roller with the flat facing the idler roller. The sheet may then enter the gap between the drive roller and the idler roller unimpeded. Once the trailing edge of the sheet has cleared the input nip, then the drive roller is rotated one revolution in the reverse direction. As the drive roller is rotated one revolution, the cylindrical portion of the drive roller contacts the idler roller, thereby pinching the sheet therebetween and driving the sheet in the reverse direction into the exit nip.
Any loss of positive gripping of the sheet by the feed mechanism during inversion, even if only very briefly, increases the reliability problems of such inverters. As the speed of the printing or copying machine is increased, the time frame after the sheet is released from the input nip within which the reversing nip roller pair must re-acquire the sheet in order to reverse the sheet""s direction becomes very short. The speed at which the reversing nip roller pair can engage the copy sheet is limited by the mass of the rollers and other factors that will be apparent to those of skill in the art. As a result, there is a maximum sheet speed beyond which the reversing nip rollers will not be able to close quickly enough to re-acquire the sheet. In which case, the sheet will clear the reversing nip and become jammed in the reversing chute.
As the overall machine speed increases, the speed at which the sheet enters and exits the reversing chute also increases. In U.S. Pat. Nos. 3,944,212 and 4,078,789 cited above, the reversing nip rotates in the reverse direction only, and must rotate more quickly as sheet speed increases in order to re-acquire the sheet and feed it into the exit nip within the available time frame. Thus, as the overall machine speed increases, the speed differential between the reversing nip and the copy sheets also increases. When the reversing nip engages the copy sheet at higher and higher speed differentials, the likelihood that the reversing nip will scuff, buckle, tear and/or otherwise damage the sheet increases.
Some devices have attempted to solve the above mentioned problem by providing a reversing nip that applies a constant reverse drive force upon the sheet that is less than the forward drive force applied to the sheet by the input nip. The drive roll in the reversing nip of such an inverter is always in contact with the idler roll and is always rotating in the reverse direction. Once the trailing edge of the sheet exits the input nip, the sheet is virtually immediately reversed by the reversing nip and driven into the exit nip. U.S. Pat. Nos. 4,359,217 and 4,346,880 are examples of such constant return force reversing nip inverters. The constant return force and friction applied to the sheet by the reversing nip drive roller in his type of arrangement, however, may scuff, buckle, tear, smear or otherwise damage the sheet, particularly as speeds increase.
Other prior art reversing chute inverters, such as disclosed in U.S. Pat. No. 4,487,506, the disclosure of which is hereby incorporated herein as of reference, provide a reversing nip roller pair in the reversing chute. Referring now to FIG. 1, the sheet enters the inverter 1 through the input nip 3 and is positively driven by the input nip into the reversing nip 5. The reversing nip, which is rotating in the forward direction at the same speed as the input nip, acquires the leading edge of the sheet (not shown) before the trailing edge of the sheet exits the input nip. Since the reversing nip 5 takes up the drive of the sheet at the same speed as the input nip, the reversing nip will not scuff, tear, mark, smear or otherwise damage the sheet. When the trailing edge of the sheet exits the input nip the reversing nip is decelerated, halted and accelerated in the reverse direction and drives the sheet in the reverse direction into the exit nip 7. The reversing nip is accelerated to the same speed as the exit nip before the leading edge of the sheet enters the exit nip. The exit nip therefore acquires the leading edge of the sheet before the trailing edge of the sheet exits the reversing nip without scuffing, buckling, tearing, smearing or otherwise damaging the sheet. This type of inverter maintains constant drive control of the sheet and relatively gently decelerates and reverses direction of the sheet.
The reversing nip described in the preceding paragraph substantially overcomes many of the sheet control and damage problems of previous reversing chute inverters. However, this type of forwarding reversing nip has a built in speed limitation. The trailing edge of a first or preceding sheet must exit the reversing nip 5 and the reversing nip must decelerate, reverse direction and accelerate to the input nip speed in the forward direction before the leading edge of a second or following sheet reaches the reversing nip. If the reversing nip 5 has not reached the same speed as the speed of the input nip 3 before the leading edge of a following sheet reaches the reversing nip, then the following sheet is likely to be scuffed, torn, buckled, smeared and/or jammed in the reversing chute.
One form of the present invention provides a method of inverting sheets traveling through a machine with an inverter having a reversing chute and a reversing nip in the reversing chute, the method comprising the steps of: a) receiving an incoming sheet into the reversing chute; b) reversing the direction of travel of the incoming sheet with the reversing nip, and driving the previously incoming sheet, which is now an outgoing sheet, out of the reversing chute; c) opening a gap in the reversing nip, while the outgoing sheet still extends through the gap in the reversing nip; d) receiving a subsequent incoming sheet into the reversing chute and through the gap in the reversing nip, while the outgoing sheet still extends through the gap; e) closing the gap in the reversing nip after the outgoing sheet has exited the reversing nip, thereby acquiring drive of the subsequent incoming sheet; and f) reversing the direction of travel of the subsequent incoming sheet with the reversing nip, and driving the subsequent incoming, which is now an outgoing sheet, out of the reversing chute.
Another form of the present invention provides a sheet inverter for inverting sheets traveling along a sheet path in a machine, the inverter comprising: a sheet reversing chute for receiving an incoming sheet; a reversing drive nip, formed of a drive roller abutting an idler roller, located in the reversing chute, such that the reversing nip reverses the incoming sheet""s direction of travel and drives the previously incoming and now outgoing sheet in the reverse direction out of the reversing chute; and a reversing nip gap device for opening a gap between the drive roller and idler roller before the outgoing sheet has exited the reversing nip, such that a subsequent incoming sheet may pass through the gap in the reversing nip while the outgoing sheet still extends through the gap; wherein the gap device closes the gap after the outgoing sheet has exited the reversing nip, such that the reversing nip reverses the subsequent incoming sheet""s direction of travel and drives the previously incoming and now outgoing sheet in a reverse direction out of the reversing chute.